1. Field
Embodiments described herein relate generally to a solid state imaging device and a method for manufacturing the solid state imaging device.
2. Background Art
When forming a transparent insulation film on a top face of a light sensing part in a back side illumination CMOS (Complementary Metal Oxide Semiconductor) sensor, it is demanded to form the transparent insulation film at low temperatures in the range of approximately 200 to 300° C. because of constraints imposed on the temperature by a bonding agent used for sticking together and Cu interconnections and it is difficult to form a good quality transparent insulation film on the top face of the light sensing part.
Therefore, the transparent insulation film formed at low temperatures has defects in an interface to the light sensing part and a high interface level. Even in a state in which there is no incidence light, electrons caused by them are detected as a current (called dark current) and appear as noise in image pickup.
In a conventional solid state imaging device proposed as a method for preventing the dark current, a negative fixed charge layer is formed on the top of the light sensing part and holes are attracted to the light sensing side of the light sensing part to form a P+ layer on the top of the light sensing part.
According to the solid state imaging device, electrons generated by a cause such as the interface level are canceled by holes because there is the P+ layer on the light sensing side, and the dark current can be reduced.
HfO2 or the like used as the negative fixed charge layer is premised on that it is formed as a film at low temperatures, however, it is difficult to reduce the defects or interface level.
Furthermore, HfO2 or the like has a high refractive index as compared with SiO2 or SiN used in the transparent film. As a result, the reflectance increases. In a structure having a negative fixed charge layer, therefore, there is a possibility that it will not be able to cope with the conventional transmission film design and limits will be imposed on the final design as well.
Furthermore, a solid state imaging device in which holes are attracted by using a ferroelectric substance instead of the negative fixed charge layer is proposed.
The solid state imaging device includes a plurality of pixels having photoelectric converters, a ferroelectric film which is formed over a photoelectric converter of each pixel via an insulation film and which is formed of an inorganic compound subjected to polarization processing, and a transparent electrode formed on the ferroelectric film.
According to this configuration, effects similar to those of the negative fixed charge layer can be obtained by retaining the polarization of dipoles of the ferroelectric substance even in a state in which voltage is not applied and adjusting the polarization direction to direct the negative charge side toward the light sensing plane.
As for an inorganic ferroelectric substance, however, heat treatment of at least 300° C. is typically needed for film forming and crystallization and its Curie point is high. It is difficult to demonstrate the performance while satisfying the temperature constraints.
Furthermore, it is difficult to use a ferroelectric substance such as barium titanate as a negative charge layer because it has a high refractive index and high temperature processing is needed to conduct sufficient polarization.